igneous
Weathered basalt cobble
Basalt; fine-grained plagioclase–pyroxene rock, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, dull to faintly subvitreous, fine-grained and rounded by abrasion; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The rusty brown areas likely reflect surface weathering and iron oxidation.
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Physical properties
Dark gray-brown, dull to faintly subvitreous, fine-grained and rounded by abrasion; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The rusty brown areas likely reflect surface weathering and iron oxidation.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and ancient lava flows. Its exact age cannot be determined from the photograph; the rounded shape indicates transport in a stream, beach, or glacial deposit.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and occasionally building stone. This ordinary cobble has little jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms columnar joints. Weathering can produce brown iron-oxide coatings and soften the surface texture.
Field identification & locations
Identify it by its heavy feel, dark fine-grained appearance, rounded cobble shape, and lack of obvious quartz or large crystals; a streak, fresh broken surface, and magnet test would help confirm it. Common worldwide, especially near volcanic terrain and river gravels.
